tree/: Convert flashchip write func ptr to enumerate

This forges the way for flashchips.c to be pure declarative
data and lookup functions for dispatch to be pure. This
means that the flashchips data could be extracted out to
be agnostic data of the flashrom code and algorithms.

TEST='R|W|E && --flash-name' on ARM, AMD & Intel DUT's.

Change-Id: I80149de169464b204fb09f1424a86fc645b740fd
Signed-off-by: Edward O'Callaghan <quasisec@google.com>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/66782
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nikolai Artemiev <nartemiev@google.com>
Reviewed-by: Felix Singer <felixsinger@posteo.net>

tests/spi25.c

@@ -32,7 +32,7 @@ struct flashchip mock_chip = {
 	.page_size	= 256,
 	.tested		= TEST_BAD_PREW,
 	.probe		= PROBE_SPI_RDID,
-	.write		= NULL,
+	.write		= NO_WRITE_FUNC,
 };
 
 /*